Conventionally known display devices include liquid crystal display devices adopting a liquid crystal display element which includes a liquid crystal sandwiched between a pair of substrates, each having at least electrodes, wherein a display is performed based on an optical response of the liquid crystal by selectively applying a voltage to the electrodes. For beneficial feature which enables a thin structure, earnest researches have been made on such liquid crystal display devices as the best candidate for the flat panel display in practical applications.
A ferroelectric liquid crystal as an example of such liquid crystal has excellent characteristics in its memory effect, high speed response, wide viewing angle, etc., which permits high precision and large capacity display using a simple matrix system [N. A. Clark and S. T. Lagerwall: Appl. Phys. Lett., vol. 36(1980)899]. FIG. 30 is a cross-sectional view schematically showing one example of the conventional structure of a ferroelectric liquid crystal display device.
Conventional ferroelectric liquid crystal display device includes two glass substrates 122a and 122b. On the surface of the glass substrate 122a, a plurality of transparent signal electrodes 123a made of indium tin oxide (generally abbreviated as ITO), etc., are formed in parallel. Further, a transparent insulating layer 124a is formed on the entire surface of the glass substrate 122a so as to coat the plurality of transparent signal electrodes 123a.
On the other hand, on the surface, facing the signal electrodes, of the other glass substrate 122b, a plurality of transparent scanning electrodes 123b made of ITO, etc., are formed in parallel in a direction orthogonal to the signal electrodes 123a. These scanning electrodes 123b are also coated with a transparent insulating layer 124b made of SiO.sub.2, etc.
On the transparent insulating layers 124a and 124b, alignment layers 125a and 125b having applied thereto uniaxial alignment treatment by the rubbing process, etc., are formed respectively. For these alignment layers 125a and 125b, organic polymeric layers such as polyimide layers, nylon layers, polyvinyl alcohol layers, etc., or SiO oblique vaporation layers, etc., may be used. In the case of adopting the organic polymeric layers for the alignment layers 125a and 125b, respective alignment treatments are applied in such a manner that liquid crystal molecules are aligned parallel to electrode substrates.
Hereinafter, the glass substrate 122a having formed thereon the signal electrodes 123a, the transparent insulating layer 124a, and the alignment layer 125a in this order is defined as an electrode substrate 120. Similarly, the glass substrate 122b whereon the scanning electrodes 123b, the transparent insulating layer 124b and the alignment layer 125b are laminated in this order is defined to be an electrode substrate 121.
The electrode substrates 120 and 121 are connected together by a sealing agent 126 except for a part which serves as an injection opening, to allow a ferroelectric liquid crystal 127 to be injected therethrough into the spacing formed between the alignment layers 125a and 125b. After the ferroelectric liquid crystal 127 has been injected, the injection opening is sealed with a sealing agent 130.
The electrode substrates 120 and 121 thus connected are sandwiched between polarization plates 128a and 128b. The polarization plates 128a and 128b are placed such that respective polarizing axes cross at right angle. In the case of a large display area, peripheral spacers 129 are dispersed so that the electrode substrates 120 and 121 are placed in parallel opposing each other with a predetermined cell thickness.
As shown in FIG. 31, a molecule 30 of the ferroelectric liquid crystal has a spontaneous polarization 27 perpendicular to a major molecule axis. Therefore, the molecule 30 of the ferroelectric liquid crystal rotates on a conical locus 28 by receiving a force in proportion to a vector product of (1) an electric field generated from a voltage applied across the signal electrodes 123a and the scanning electrodes 123b and (2) the spontaneous polarization 27.
Therefore, when seen from an observer, the molecule 30 of the ferroelectric liquid crystal switches between the position A and the position B of the axes of the conical locus 28. Here, for example, by arranging such that one polarization axis of the polarization plates 128a and 128b coincides with a major molecular axis direction 29a in the state the molecule 30 is switched to the position A, and the other polarization axis coincides with a direction 29b, a dark view can be achieved. On the other hand, when the molecule 30 is switched to the position B, a bright view can be achieved by birefringence.
The respective alignment states of the molecule 30 of the ferroelectric liquid crystal in the positions A and B are equivalent in terms of elastic energy. Therefore, even after the electric field is removed by the signal electrodes 123a and the scanning electrodes 123b, the alignment state and the optical state of the molecule 30 can be maintained, which is known as a memory effect of the ferroelectric liquid crystal. The described memory effect cannot be achieved from the conventional nematic liquid crystal, and the memory effect and the high speed response characteristic by the spontaneous polarization enable the ferroelectric liquid crystal display device to offer high precision and large capacity display by the simple matrix method.
In general, in the large-size liquid crystal display device, a deformation of the substrate is likely to occur due to externally applied forces such as a buckling due to a weight of the substrate itself, an impact, etc. When a thickness between the substrates opposing one another varies by a deformation of the substrates due to externally applied pressure, the alignment of the liquid crystal molecules is disturbed, and irregularities in threshold voltage are likely to occur due to a leakage of the electrode, thereby presenting a problem that a desirable display is difficult to be achieved.
In order to counteract the described problems, either one of the following methods adopting a spacer for maintaining a uniform spacing between the substrates has been adopted: (1) A method of dispersing spherical particles; and (2) A method of forming a wall in a pillar shape of an organic or inorganic series.
However, the described method (1) has the following drawbacks. Firstly, as the agglomeration of the fine particles is likely to occur, it is difficult to disperse fine particles uniformly on the substrate, which, in turn, makes it difficult to achieve a uniform cell thickness. Secondly, as it is difficult to control the position of the particles, the disturbance of alignment is likely to occur due to the particles dispersed in the pixel region, which results in the deterioration of the display quality. Thirdly, in the method (1), the substrate is supported only by a fulcrum of the spacer, and the substrates are not connected together by the spacer, a precise control of the spacing between the substrates is difficult to be achieved, and a sufficient strength for maintaining a spacing between the substrates against the externally applied pressure cannot be obtained.
As an example of the method (1), a method of dispersing adhesive particles and spacer beads simultaneously has been proposed (for example, by Japanese Unexamined Patent Publication No. 174726/1987 (Tokukaisho 62-174726)). However, in order to ensure sufficient adhesiveness for practical applications, it is required to disperse the adhesive particles and the spacer beads at high density. Moreover, a display quality may be lowered due to the spacer as dispersed.
According to the method (2), a spacer is formed in a form of a pillar shape by the photolithography using an organic or inorganic film. In this method, as a pole can be selectively formed outside the pixel region, a contact surface between the substrate and the pole can be controlled as desired. The described method (2) is superior to the aforementioned method (1) as it offers a solution to the described three problems associated with the method (1).
As an example of the method (2), Japanese Unexamined Patent Publication No. 257824/1989 (Tokukaihei 1-257824) discloses a method of forming a resin spacer material in a shape as desired, for example, by the photolithography for example.
Recently, for the liquid crystal material, the described ferroelectric liquid crystal has been viewed with interest. The ferroelectric liquid crystal has excellent characteristics such as high speed response achieved by its spontaneous polarization, or being independent of a viewing angle by a switching on the plane. In contrast, however, as the ferroelectric liquid crystal has a still closer molecule regularity to crystal, if the molecular regularity is disturbed by an externally applied pressure, an original state cannot be attained. In other words, the ferroelectric liquid crystal does not have a sufficient resistance to impact.
For the described reason, for the spacer designed for the liquid crystal display element adopting the ferroelectric liquid crystal, the method (2) is considered to be the most effective method. To be specific, the following methods are known: (a) after forming the alignment control layer of the polyimide type or polyamic acid type which is completely imidized, the spacer is formed in its upper layer and (b) after forming the spacer, the alignment control layer is formed and subjected to the rubbing process so as to combine the substrates with each other.
However, as described, in the case where the resin spacer material is formed on the alignment control layer, the following problem arises. Namely, since the alignment control layer is soiled by (1) a solvent for use in applying the spacer material on the alignment control layer, (2) the resin as the spacer material itself, and (3) the developer for use in the photolithography, the effects of the rubbing process applied to the alignment control layer are lowered, or the alignment control force with respect to the liquid crystal is lowered.
In order to counteract the described problem, an attempt is made to apply a rubbing process after the spacer is formed. However, when adopting this method, as the rubbing process is applied also to the spacer itself, a uniaxial alignment of the polymeric chain is formed on the surface of the spacer. As a result, as the liquid crystal is aligned unnecessary, another problem arises in that an abnormal alignment or a switching inferior occurs in the liquid crystal in a vicinity of the spacer.
The liquid crystal display element including a spacer in a pillar or wall shape manufactured by the conventional method has drawbacks in that an adhesive force is not generated between the upper and lower substrates, or these substrates as connected together are easy to come off.
For example, in the case of forming the alignment control layers, for example, by an imide compound such as polyimide resin, etc., it is difficult to connect respective alignment control layers made of polyimide resin together. For this reason, the polyimide resin has low response characteristics and is determined to be a relatively hard layer as a polymer layer.
When laminating the substrates by forming a spacer in a pillar shape on the alignment control layer formed on one of the substrates and making the other substrate adhere to the spacer, the adhesiveness can be achieved to some degree by adopting an adhesive resin for the spacer; however, a sufficient strength cannot be achieved, and the two substrates as laminated are likely to come off.
Moreover, in the case of adopting an organic or inorganic resin which does not apply the adhesiveness to the spacer, the upper and lower substrates cannot be connected together.
As described, an insufficient adhesiveness between the upper and lower substrates causes cell thickness deviations, which, in turn, cause a deterioration of display characteristics. Moreover, an unwanted spacing is formed between the upper and lower substrates, which allows the movement of the liquid crystal, and significantly lowers the resistance to externally applied pressure.
In order to counteract the described problem, the method of making the described spacer in a wall or pillar shape adhere to the substrate by an adhesive has been proposed. However, in the case of adopting the adhesive, if the adhesive is protruded from the pixel section, the adhesive as protruded would disturb the alignment of the liquid crystal, which give raise to another problem of the deterioration of the display quality.
For example, the method of forming the adhesive layer which serves as the spacer on the surface of the substrate (for example, as disclosed by Japanese Unexamined Patent Publication No. 116126/1988 (Tokukaisho 63-116126) is considered to be effective for improving the adhesive characteristics as well as for precisely controlling a spacing between the substrates. However, the described methods do not offer a solution to the problem that the alignment control layer is soiled in the process of forming the spacer layer on the alignment control layer, and thus the disturbance of the alignment of the liquid crystal remains unsolved.
Another arrangement wherein the adhesive layer is formed only on the spacer has been proposed as a solution to the problem of soiling the alignment control layer (see SID 93 Digest, p961-964). However, in this method, as the plastic film is adopted for the substrates, and the glass substrate which is the most generally used for the substrates of the liquid crystal display element cannot be used. Thus, a device for transferring or applying the adhesive on the substrates with a precision of sub micron order is required, and thus it is not suited for practical applications.
The conventional liquid crystal display element also has other drawbacks as will be explained through another example of the conventional liquid crystal display element in reference to FIG. 33.
As shown in FIG. 33, the conventional liquid crystal display element (liquid crystal cell) has a pair of light transmissive substrates 131 and 132 wherein a plurality of electrodes 133 and a plurality of electrodes 134 are formed respectively in a stripe shape. The electrodes 133 are formed in a direction orthogonal to the electrodes 134.
The electrodes 133 are entirely coated with an insulating layer 135 and an alignment layer 136, and light shielding members 137 are formed on both sides of each electrode 133. The electrodes 134 are entirely coated with an insulating layer 138 and an alignment layer 139, and light shielding members (not shown) similar to the liquid shielding members 137 are formed on both sides of each electrode 134.
As described, an electrode substrate 140 is prepared by forming the electrodes 133, the insulating layer 135, the alignment layer 136 and the light shielding members 137 on the substrate 131 in this order. Similarly, an electrode substrate 141 is prepared by forming the electrodes 134, the insulating layer 138, the alignment layer 139 and the light shielding members on the substrate 132 in this order.
The described electrode substrates 140 and 141 are connected together by a sealing agent 142 with a spacing between them in such a manner that respective surfaces having formed thereon the electrodes 133 and the electrodes 134 oppose each other. In the spacing, spherical spacers 143 are formed, and the liquid crystal is injected therein, thereby preparing a liquid crystal layer 144.
The spacing between the electrode substrates 140 and 141 is extremely narrow, generally in a range of from 1 to 20 .mu.m. In order to inject the liquid crystal in such a narrow spacing, generally, either one of the following methods is adopted: (a) A method of injecting the liquid crystal under an atmospheric pressure; and (b) A method of injecting the liquid crystal under a reduced pressure.
As a concrete example of the method (a), the method of injecting the liquid crystal through a plurality of injection openings formed in the sealing agent 142 under an atmospheric pressure utilizing the capillarity is known. In this method, as air in a form of bubbles remains in the spacing after the liquid crystal is injected, the display quality deteriorates.
Specifically, the method (b) includes the first step of placing empty cells in a container a pressure of an inside of which can be reduced and reducing the pressure of the spacing formed between the electrode substrates 140 and 141 and the atmosphere, the second step of heating empty cells to or above a temperature at which liquid crystals show a nematic phase or an isotropic phase, the third step of sealing the injection opening with the liquid crystal, and the fourth step of increasing the pressure to the original atmospheric pressure. In the described method, as the injection opening is sealed with the liquid crystal in the third step, the pressure in the spacing between the substrates can be kept at a reduced pressure even after increasing the pressure back to the atmospheric pressure in the fourth step, thereby causing a difference in pressure from the atmosphere.
As described, according to the described method (b), as liquid crystals are deformed between the substrates by reducing pressure, an amount of residual foams can be reduced compared with the case of adopting the method (a), and the method (b) offers superior display quality to that achieved by the method (a).
The inventors of the present invention has examined the effects of an angle formed by the injection direction and the rubbing direction on alignment characteristics, and have found that when varying the angle for each pixel, a uniform alignment cannot be obtained.
As shown in FIG. 34(a) and FIG. 34(b), in the conventional injection method, when injecting a liquid crystal 115 through an injection opening 116, the liquid crystal is spread in a fan shape. Therefore, it is difficult to control an angle formed by the injection direction and the rubbing direction. As described, in the conventional method, as the angle differs for each pixel, the liquid crystal 115 becomes in disorder, which, in turn, may impair a display quality.
Moreover, in the case of injecting the liquid crystal in the manner shown in FIG. 34(a) and FIG. 34(b), a long time is required. For this reason, when injecting the liquid crystal by the method (b) using a liquid crystal compound of low boiling point, after the liquid crystal composition is placed under reduced pressure for a long period of time, the liquid crystal volatizes, thereby presenting the problem that the liquid crystal composition deviates. On the other hand, in the liquid crystal display element of FIG. 34(b) having a plurality of injection openings 116, the liquid crystal 115 does not move into a vicinity of both ends of the sealing agent 142 close to the injection opening 116, and a liquid crystal non injected area 117 appears. The described deficiency with regard to the injection of the liquid crystal causes the deterioration of the display quality.
On the other hand, when the adhesive force generated between the electrode substrates 140 and 141 by the sealing agent 142 is not sufficient, due to changes in temperature of the panel and the pressure in the injection process and also depending on an amount of the liquid crystal injected, the spacing between the electrode substrates 140 and 141 differs before and after the injection, thereby causing a non-uniform cell gap. As described, when the adhesive force between the electrode substrates 140 and 141 is not sufficient, a precise control of the cell gap may be difficult to be achieved.
In the conventional liquid crystal display element shown in FIG. 33, as the spacers 143 are dispersed in a spacing between the electrode substrates 140 and 141, the effect of reducing the impact is weak, and for this deficiency, the conventional liquid crystal display element shown in FIG. 33 is not suited for the ferroelectric liquid crystal.